Health & Fitness

BMI is a quick screening number computed as weight divided by height squared (kg/m², or 703 × lb/in² in imperial units) — it doesn't measure body fat directly, but decades of population data link it to broad weight categories: under 18.5 is underweight, 18.5–25 is normal, 25–30 is overweight, and 30+ is obese. It's a useful starting point for tracking trends over time, but it can misclassify very muscular or very lean individuals, so treat it as one data point rather than a diagnosis and pair it with other measurements like body fat percentage when possible.

This calculator first estimates your BMR — the calories your body burns at total rest — using the Mifflin-St Jeor equation, which combines weight, height, age, and sex (currently the most accurate widely-used formula for this). That BMR is then multiplied by an activity factor, ranging from 1.2 for sedentary to 1.9 for very active, to get your TDEE, the calories you burn in an average day. Your goal calories simply shift that TDEE by roughly 500 kcal/day up or down, since a 500 kcal daily deficit or surplus corresponds to about one pound of body weight change per week — treat these as helpful starting estimates rather than exact numbers, since individual metabolism varies.

The US Navy method estimates body fat from a handful of tape-measure circumferences — neck and waist for men, plus hips for women — plugged into a logarithmic formula that correlates well with more expensive methods like DEXA scans without needing any special equipment. Because it relies on where you carry fat around your waist and neck, it tends to track body composition changes (like losing fat while maintaining muscle) more meaningfully than the scale alone. For the most consistent results, measure at the same time of day with the tape snug but not compressing the skin, and remember this is an estimate, not a clinical measurement.

This tool uses Naegele's rule, the standard obstetric estimate that adds 280 days (40 weeks) to the first day of your last menstrual period, or 266 days if you're counting from a known conception date instead. Your current week and trimester are then derived from how many days have elapsed since that starting point, with trimesters split at roughly week 13 and week 27. Naegele's rule assumes a fairly standard 28-day cycle, so actual delivery dates commonly land within a couple weeks of the estimate either way — this is general guidance, not medical advice, so use it alongside your doctor's ultrasound-based dating.

Your max heart rate is estimated with the classic 220 minus age formula, then split into five zones by percentage — from an easy 50-60% warm-up up to 90-100% max effort. The Karvonen method refines this by using your resting heart rate to compute heart-rate reserve (max minus resting), which better accounts for individual fitness levels since two people with the same max HR can have very different resting rates. As a practical tip, most aerobic base-building happens in Zones 2-3, while Zones 4-5 are for shorter, harder intervals — these are general training guides, not a substitute for advice from a coach or doctor if you have a heart condition.

The baseline here uses a common clinical rule of thumb — about 35 ml of water per kilogram of body weight per day — then layers on extra fluid for exercise (roughly 350 ml per activity tier), hot or humid climates (an added 500 ml to offset extra sweating), and pregnancy or breastfeeding, which increase fluid needs to support the additional blood volume or milk production. This total includes water from all beverages and food, not just plain drinking water. As with most hydration guidance, thirst and urine color are good real-time indicators too, and this is general guidance, not medical advice for specific health conditions.